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PDF is a file format developed by Adobe Systems for representing documents in a manner that is separate from the original operating system, application or hardware from where it was originally created. A PDF file can be any length, contain any number of fonts and images and is designed to enable the creation and transfer of printer-ready output.

Each PDF file encapsulates a complete description of a 2D document (and, with the advent of Acrobat 3D, embedded 3D documents) that includes the text, fonts, images and 2D vector graphics that compose the document. They do not encode information that is specific to the application software, hardware, or operating system used to create or view the document.

HTML4 has been superceded by HTML5, however still plays an important role in the history of HTML. HTML4 was the first version to include cascading style sheets (css), and these were changed further in HTML5 which included more presentational elements - HTML4 led the way here.

HTML 4.01 was published in late 1999 as a W3C Recommendation. It offers three variations: Strict, in which deprecated elements are forbidden, Transitional, in which deprecated elements are allowed and Frameset, in which mostly only frame related elements are allowed. XHTML is a separate language that began as a reformulation of HTML 4.01 using XML 1.0. It is no longer being developed as a separate standard. Most presentational markup elements were deprecated under the HTML 4.0 specification in favor of using CSS for styling.